A 6-year-old boy is brought to the pediatric clinic with a 3-week history of bone pain, pallor, and recurrent infections. His mother reports he has been refusing to play and complains of leg pain at night. On examination, he has generalized lymphadenopathy, hepatomegaly, and petechiae. Complete blood count shows WBC 120,000/μL with 80% blasts, hemoglobin 6.8 g/dL, and platelets 25,000/μL. Bone marrow shows 85% blasts that are TdT+, CD19+, CD20+, CD10+, MPO−. What is the most likely diagnosis?

Explanation

## Diagnosis: Precursor B-Cell ALL, Common Type (cALL) ### Clinical Context This is a classic presentation of childhood ALL: bone pain (from marrow infiltration), pallor (anemia), infections (leukopenia/immunosuppression), and constitutional symptoms. The age (6 years) is peak incidence for ALL. ### Immunophenotype Analysis | Marker | This Case | B-ALL | T-ALL | AML | |--------|-----------|-------|-------|-----| | TdT | + | + (early B) | + | − | | CD19 | + | + (B lineage) | − | − | | CD20 | + | + (mature B) | − | − | | CD10 | + | **cALL marker** | − | − | | MPO | − | − | − | + | | CD13/CD33 | − | − | − | + | **Key Point:** The combination of **TdT+, CD19+, CD20+, CD10+, and MPO−** is pathognomonic for B-ALL, specifically the **common type (cALL)**. CD10 (CALLA antigen) is present in ~80% of childhood B-ALL and defines the common immunophenotype. ### Morphology & Cytochemistry - Blasts are **medium-sized with scant cytoplasm** (lymphoid morphology) - **MPO−** rules out myeloid differentiation (AML) - **TdT+** indicates early lymphoid precursor ### High-Yield Features of cALL **High-Yield:** Common ALL (cALL) accounts for ~60% of childhood ALL. It has: - CD10+ (CALLA+) immunophenotype - t(12;21) ETV6-RUNX1 fusion in ~25% (good prognosis) - Better prognosis than other B-ALL subtypes - Excellent response to modern chemotherapy protocols (>90% 5-year survival) ### Mnemonic **cALL = CD10+ B-ALL + TdT+ + MPO− + Childhood peak + Good prognosis** ### Clinical Pearl The presence of hepatosplenomegaly and lymphadenopathy indicates significant extramedullary involvement, which is common in ALL. The high WBC count (120,000/μL) and blast percentage (80%) confirm acute leukemia. [cite:Robbins 10e Ch 13]